I created an extension for Facebook that had 30k active users. I sold it after I was contacted like this. I was suspicious immediately from the outset about who the buyer was, but I figured I'd sell anyway as it took me about an hour to write. Strangely the app was deleted from the store within an hour of my purchase and has yet to re-appear: https://chrome.google.com/webstore/detail/facebook-chat-fix/...